The present invention relates to a method of forming a groove in a valve seat so as to allow passing of a small amount of fluid between the valve seat and a valve member even if the valve member is abutted against the valve seat, and to a tool used for this method.
Such a groove is formed, e.g., in a valve seat of an expansion valve used in a refrigeration cycle.
When the refrigeration cycle is stopped, the operation of a compressor is stopped and the supplying of a control signal to a valve opening control means of the expansion valve is also stopped. Therefore, when a groove as described above is not formed in the valve seat, high-pressure coolant is trapped between the expansion valve and the compressor. The high-pressure coolant, when trapped, imposes a load on the compressor when the refrigeration cycle is resumed and adversely affects the operation of the compressor. In a conventional method of forming a groove to prevent this problem, a tool having a pyramid-like distal end with a hardness greater than that of the surface of the valve seat is used.
In the conventional method of using such a tool, if the tool is off-centered or misaligned from the valve seat or if the stroke of the tool is not properly controlled, the size and shape of the formed groove will be different from the design. Then, the time period required for equalizing the pressure in a passage between the expansion valve and the compressor (to be referred to as an upstream passage), i.e., a passage under high pressure, to that of the coolant in a passage downstream from the expansion valve, that is, a passage under low pressure, from the time of closing the expansion valve, will deviate from the time period determined by the design of the refrigeration cycle. That is, the refrigeration cycle will be resumed before the pressure of the coolant in the upstream passage reaches a balance with that of the coolant in the downstream passage. As a result of this, the compressor may be damaged. However, if a high-precision requirement is imposed on the centering of the valve seat and the tool and the stroke of the tool, the conventional method of forming a groove in a valve seat is rendered complex in its procedure and the manufacturing cost is increased. Furthermore, in this conventional method, when the distal end of the tool wears, it must be reground to a desired size and shape irrespective of the relatively high level of hardness of the distal end. This grinding operation is complex in procedure and costly to accomplish.